The marine propulsion industry has changed through recent years. On small boats, the use of outboard motors are the most popular but have certain limitations, namely the stern of the boat is consumed by the outboard motor. This position has multiple drawbacks for fisherman including accessing the transom of the boat for swimming or fishing. Boats with an outboard have limited space for use of a swim platform. If the boat has two, three, or four outboards, the transom of the boat is essentially non accessible. Stern drive propulsion units have also been popular but, similar to an outboard, the position of the propeller at the back of the boat makes them less desirable for fisherman, swimmers and divers. Also, the positioning of the propeller at the back of the boat has certain steering limitations as the thrust from the propeller is used to move the boat.
Inboard positioned propulsion systems found on larger recreational boats, including diesel engine driven vessels, have seen competition from “Pods”. Pods are a relatively new development, initially brought to the recreational yachting market by Volvo, and followed by Mercury and Twin Disc. They are large drive propulsion units that are installed under the bottom of the boat which steer the propellers and drives for better handling than inboards with rudders. Further, Pods typically include counter-rotating propellers with improved efficiency. The installation of Pods requires substantial changes to the boat bottom and general layout of other features within the boat.
One other significant improvement in boat performance is the use of surface piercing propellers. Counter-rotating propellers have also been used to achieve the benefit of both the surfacing propeller technology and counter rotation together for maximum performance. Surface piercing propellers are generally found on drives with propellers in back of the transom because they require a huge amount of ambient air for the upper part of the propeller.
The Applicant holds a number of patents for a technology marketed as Power Vent®, including U.S. Pat. Nos. 6,045,420; 6,193,573; 6,213,824; 6,325,010; 6,390,776; and U.S. Pat. No. 6,461,206. This technology positions a surface piercing propeller beneath the hull of an inboard powered boat with the propeller vented to atmosphere through a vent to the transom. This technology has been proven in boats with over 2,000 horsepower at speeds of over 50 knots. However, low speed handling with rudders is not as good as a Pod drive, and larger boats with a single drive stem require a bow thruster docking.
It is also problematic to put counter-rotating steering drives under boat hulls because they require a large space under the boat. The drives also require significant vertical space and result in a deep water draft limiting where these boats can go. Outboards can tilt upward for swallow water use. Stern Drives can also tilt but to a much less degree than the outboards. Surfacing drives require only one-half of the propeller to be in the water, further improving shallow water capability.
What is lacking in the industry is a propulsion system having a steering function that allows placement in confined areas, such as a vented or non-vented boat hull tunnel.